The news just broke that @stroughtonsmith got Siri working on his jailbroken iPhone4. Nice. There is a vid, will post a link to it, then do an embed later. Right now he is not releasing the files or steps till the iPhone4s jailbrake is out there. Also, he wants to do a bit more testing
But he says it works fine and from what I saw on the video, there was no issues with the slower CPU.